Materials such as zinc sulfide are highly desirable materials for infrared (IR) articles, such as windows and domes for high speed aeronautical vehicles which may reach transonic speeds, due to their high transmission in the visible to long wavelength infrared (LWIR) band region, i.e., 0.6 μm to 14 μm. In general, transmissions through zinc sulfide may be from around 60% and greater. However, zinc sulfide is also relatively soft which makes it unsuitable for high speed aeronautical vehicles. Such articles must withstand rain and sand impact, as well as provide high transmission in the required wavelength bands. However, zinc sulfide typically suffers considerable damage under rain and sand impact which results in loss of transmission and substantial increase in scatter. Scatter is a general physical process where radiation, such as light or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In general, rain erosion tests are conducted in an artificial rain field of 2 mm nominal drop diameter falling at a rain rate of around 25.4 mm/hr and impacting samples at a velocity of around 210 m/sec. for an exposure time of around 20 minutes. A typical test is the Whirling Arm Rain Rig test performed by the University of Dayton Research Institute at the Wright Patterson Air Force Base, Dayton, Ohio. Tests performed on zinc sulfide have indicated that zinc sulfide suffered considerable damage when exposed to the above rain field for 5 minutes or longer.
To improve durability of zinc sulfide, coatings of hard and durable materials, such as diamond like carbon, alumina, boron nitride and gallium phosphide are applied on infrared windows of zinc sulfide. The choice of coating material depends upon the particular transmission band of interest. However, coatings may suffer from adhesion problems as well as not being uniform over the surface of the zinc sulfide article. Variation in size of the article as well as shape such as unconventional angles on the surface can make application of a coating difficult. A number of physical properties of the materials are involved when coating thicknesses are increased or when the size of the article on which the coating is deposited is changed. Typically stresses, such as tensile and compressive stress, are involved which are unpredictable in their effects as the coating thickness is changed as well as the size of the article is changed. Accordingly, there is a need for increasing the hardness of zinc sulfide which can survive the full duration of rain and sand erosion and transmit in the required wavelength bands and are durable for use in high speed aeronautical vehicles.